The present invention relates to a laser cutting method which utilizes the energy of a laser beam to cut various materials, and more particularly, to a laser cutting method effective for preventing deterioration of the cut surface of a workpiece due to changes in cutting conditions and for preventing degradation of the cutting quality of the workpiece when the cutting conditions are changed during laser cutting.
Cutting conditions are often changed during laser cutting of a sharp corner or the like, which is difficult to cut well under conditions of high speed and high output (hereinafter referred to as "first cutting conditions") used for ordinary straight cutting. Conventionally, to cut a sharp corner or the like, the cutting conditions were changed to those of low speed and low output (hereinafter referred to as "second cutting conditions") under which only a range of several millimeters before and after the corner could be cut well without interruption of laser impingement.
FIG. 25 illustrates such a conventional cutting method and shows a cutting path employed with a cutting condition changing method such as that disclosed in Japanese Laid-Open Patent Publication No. Sho. 63-63593. In this conventional approach, as shown in FIG. 25, a workpiece is first cut under the first cutting conditions, which are then changed to the second cutting conditions at point A on the cutting path with laser impingement maintained, and a corner is cut under the second cutting conditions, which are then changed back to the first cutting conditions at point B on the cutting path.
FIG. 26 shows another conventional approach disclosed in Japanese Laid-Open Patent Publication No. Hei. 2-30388. In this approach, the cutting speed under the second cutting conditions is changed in stages, i.e., 10%, 20%, 40%, 60% and 100% of the cutting speed under the first cutting conditions, during time period T at intervals T1, T1+T2, T1+T2+T3, and T1+T2+T3+T4 in returning from the second cutting conditions to the first cutting conditions.
FIG. 27 illustrates another cutting method, showing an example of switching conditions on only one side of a corner, as disclosed in Japanese Laid-Open Patent Publication No. Sho. 60-127775 and Japanese Laid-Open Patent Publication No. Hei. 5-277773. In this method, the first cutting conditions are used for cutting up to T2 at the point of the corner, the second cutting conditions are used for cutting between T2 and Tp, and the first cutting conditions are used again for cutting from T3.
As yet another example of a conventional approach, Japanese Laid-Open Patent Publication No. Hei. 3-106583 discloses a corner cutting method in which, after laser cutting is conducted up to a corner, a cooling medium is injected for a preset period of time to cool the workpiece, and cutting is then resumed.
However, when the cutting condition changing method shown in FIG. 25 was carried out, as shown in FIG. 21, chipping, gouging or the like of a part of a workpiece in the cut surface of the workpiece tends to occur near point A where the cutting conditions are changed, resulting in a degraded cutting quality of the cut product. The causes of such a problem are sudden changes in cutting speed, cutting laser output, etc., due to the changes in cutting conditions during cutting, and the resultant disturbance of the flow of the cutting gas (gas injected in the direction of the same axis as the laser beam for such purposes such as removal of a molten area from the workpiece and acceleration of an oxidative combustion reaction in laser cutting). Also, heat concentration near the cut surface of the workpiece due to laser cutting is a factor in the increase of such gouging.
FIG. 21 is a sectional view showing the state of cutting after the conditions have been changed at point A during cutting. In this drawing, reference numeral 1 indicates a laser beam and 3 denotes a workpiece. Generally, when conditions are changed during cutting, chipping occurs at the position where the conditions were changed. This is caused by the deviation of the actual cutting position at the position of impingement of the laser beam (deviation "m" in FIG. 21), resulting in abnormal combustion at the point where the cutting conditions are changed. Hence, it is expected that no gouging is produced at point B where the cutting condition switching position occurs at a point prior (retraction distance "l") from the deviation "m", as shown in FIG. 22. FIG. 23 indicates deviations "m" produced when soft steel materials 12 mm and 19 mm thick were cut with the cutting speed changed. As described above, deviations "m" depend on material thickness and cutting speed. The cutting conditions of the above materials are indicated in Tables 1(A) and 1(B).
TABLE 1 (A) ______________________________________ Conditions for 12 mm Thickness Speed (m/min) Output (W) Other Conditions ______________________________________ 0.4 700 Cutting lens: f10" 0.6 1000 Assist gas pressure 0.7 kg/cm.sup.2 0.8 1400 Focal position: +1.5 mm 1.0 1800 1.2 2300 ______________________________________
TABLE 1 (B) ______________________________________ Conditions for 19 mm Thickness Speed (m/min) Output (W) Other Conditions ______________________________________ 0.4 1000 Cutting lens: f10" 0.6 1400 Assist gas pressure 0.7 kg/cm.sup.2 0.8 1800 Focal position: +1.5 mm 1.0 2400 1.2 2900 ______________________________________
The other primary cause of chipping is uneven heat distribution in the periphery of a cut groove during cutting. As the ambient temperature of the cut groove is higher at the time changing the cutting conditions, the heat conduction is greater, leading to a higher probability of abnormal combustion. In FIG. 24, reference numeral 2 designates a cut groove and reference numeral 1 represents a laser beam. A temperature distribution shown in FIG. 24 is produced around the cut groove. A cutting fault is likely to occur under such conditions, especially at a temperature of not less than approximately 500.degree. C. As the workpiece becomes thicker, the ambient temperature is higher and more time is required for cooling.
In the case of materials other than soft steel, melting faults do not generally occur at the condition changing position. In the case of soft steel, however, melting faults can easily occur at the condition changing position. The oxidation reaction of the soft steel material is indicated by the following reaction equations (or their combination): ##EQU1##
Thus, excessive heat is generated, in addition to the energy of the laser beam, and hence melting faults occur. In the case of nonferrous metals, however, there is no significant heat of reaction, and thus melting faults do not occur.
In the conventional approach shown in FIG. 26, high quality is achieved when a relatively thin, e.g., 6 mm or less, workpiece is cut at low speed, e.g., 1 m/min or less, but as the workpiece becomes thicker and the cutting speed higher, a cutting fault is more likely to occur at the point where the conditions are changed.
Further, the conventional approach shown in FIG. 27 is effective for preventing a molten portion of a corner from dropping away due to chipping caused by cutting deviation and accumulated heat decreases due to cutting conditions of low speed and low output between T2 and Tp. However, molten area dropping, as described with regard to FIG. 21, occurs at the condition changing position Tp, reducing the cutting quality as a whole.